The present invention relates generally to the aqueous emulsification of high molecular weight polyolefins using a direct pressure process. In particular, the present invention relates to the aqueous emulsification of functionalized or chemically modified polyolefins that have a molecular weight greater than 10,000 in a one-step direct pressure process. The present invention also relates to the direct application of these high molecular weight functionalized polyolefin emulsions onto glass fibers, either during the glass fiber manufacturing process or at a later stage, to obtain reinforced polypropylene composites with a high mechanical performance.
It is known in the art that glass fiber reinforced polymer composites possess higher mechanical properties compared to unreinforced polymer composites, provided that the reinforcement fiber surface is suitably modified by size chemical formulation. Thus, better dimensional stability, tensile strength and modulus, flexural strength and modulus, and impact resistance and creep resistance can be achieved with glass fiber reinforced composites.
Glass fiber reinforced polypropylene (PP) composites have widespread applications in various market sectors such as automotive, household, and other electrical appliances, that require a combination of specific short- and long term mechanical, physical, chemical, aging, and aesthetic properties. These properties play an important role in designing the final composite part. For example, stronger polypropylene composites permit the formation of parts with thinner walls, which helps to improve productivity with reduced cycle time, to reduce the weight of the part, to reduce the materials used to make the part, and to reduce the cost of the part. Improving the strength of composites also helps extend the life of the final part. In view of emerging application demands, the polypropylene composite industry is constantly looking for ways to develop stronger xe2x80x9cnext generationxe2x80x9d polypropylene composites for new market applications and for replacements of other more expensive engineering plastics currently in use.
It is also known in the art that fiber-matrix interface interactions influence many bulk mechanical properties of reinforced composites. Thus, to effectively transfer the applied load from a weaker matrix resin to stronger fibers, it is necessary to improve fiber-matrix interactions, especially in glass fiber reinforced thermoplastic composites. Fiber surface treatment by applying chemical sizing formulations during glass fiber manufacturing to modify the fiber surface and improve fiber-matrix interactions, adhesion, and compatibility in composites has been practiced in the industry.
Various aqueous sizing formulations have been used in the glass fiber industry to maximize the fiber-matrix interactions for polypropylene composites. These sizing formulations include ingredients that collectively form an interphase between glass fibers and the matrix resin. Typically, the sizing formulation includes ingredients such as a film forming resin, a silane, a lubricant, an antistatic agent, and other chemical ingredients. Sizing formulations that include an aqueous emulsion of chemically modified or functionalized polyolefins, such as maleic anhydride grafted polypropylene, have been found to be beneficial.
The maleic anhydride grafted polypropylene ingredient included in most conventional sizing formulations in the form of an aqueous emulsion possesses a very low molecular weight (i.e., a molecular weight 6,000-9,000) and high grafting functionality levels (i.e., 5-10% by weight). The lower molecular weight (i.e., a molecular weight less than 10,000), the lower melt viscosities, and the higher maleic anhydride functionalization of these grafted polypropylenes have enabled their emulsification, such as by xe2x80x9cindirect pressurexe2x80x9d or xe2x80x9cdirect pressurexe2x80x9d methods, without much difficulty. For example, a low molecular weight polyolefin is typically melted together and mixed with suitable emulsifying agents. An emulsion is then obtained by adding the necessary amount of water.
One example of a low molecular weight grafted polypropylene that is readily available is Epolene E43, a homopolypropylene grafted with maleic anhydride having a weight average molecular weight of approximately 9100. An aqueous emulsion from this grafted polypropylene has been useful in glass fiber sizing applications when it is a major ingredient. However, it is believed that because of the low molecular weight of the grafted polypropylenes, the composites reinforced with glass fibers sized with such low molecular weight grafted polypropylene formulations are not strong enough to meet current application needs. To enhance the lower end properties of such polypropylene composites, it has become common practice to add a high molecular weight functionalized polypropylene in solid form during the compounding stage of the manufacturing process. However, high quantities in solid form must be added to compensate for these lower end properties (e.g., between 2-15% by weight of the matrix resin must be added). Moreover, during the compounding stage, the added high molecular weight grafted polypropylene is dispersed throughout the composite part and is only partially directed towards the fiber surface, which results in a non-optimal use of this generally more expensive grafted solid polypropylene additive.
The aqueous emulsification of such high molecular weight grafted polypropylenes is difficult due to their higher molecular weight, higher melt viscosity, lower melt flow rate (MFR) or melt flow index (MFI), higher hydrophobicity, and relatively lower polarity. The emulsification of isotactic, high molecular weight grafted polypropylene becomes even more difficult due to their higher tendency to crystallize. Thus, it is extremely difficult to derive formulations to successfully achieve an aqueous emulsification of high molecular weight grafted polypropylene.
Various techniques have been disclosed to emulsify high molecular weight functionalized polyolefins. For example, French Patent No. 2,588,263 describes a technique for emulsifying isotactic polyolefins of high molecular weight by dissolving the polymer with heat in an organic solvent that is immiscible in water. Water is then added to dilute the mixture. This process requires the subsequent elimination of the solvent by extraction or by washing and drying. In addition to the burden of having additional steps, the use of organic hydrocarbon solvents creates safety concerns for the chemist emulsifying the polyolefins.
U.S. Pat. No. 4,240,944 describes the co-emulsification of a mixture of a high molecular weight isotactic grafted polypropylene together with a lower molecular weight amorphous grafted polypropylene in a ratio of 1:1 to 1:4 parts by weight along with the base and surfactant and subsequent addition of water to obtain an emulsion. However, in this method, not more than 50% of the isotactic high molecular weight grafted polypropylene can be incorporated into the emulsion. Further, it is believed that fairly large concentrations of lower molecular weight amorphous grafted polypropylene may ultimately be detrimental for composite properties at room temperature and, particularly, at elevated temperature applications.
U.S. Pat. Nos. 5,242,969 and 5,389,440 describe a two-step method of forming a high molecular weight polypropylene aqueous emulsion. In the first step, fluidization, melt mixing, and melt blending of a high molecular weight grafted polypropylene with a sufficient quantity of fatty acid is accomplished in an extruder at high shear and high temperature. The mixture is then cooled and ground. In the second step, the mixture is combined with a base and other ingredients in a pressure reactor. This method is disadvantageous in that it requires two steps, is expensive, and causes the polypropylene resin to experience two thermal cycles, which leads to excessive degradation and deterioration of the polypropylene structure. This degradation and deterioration of the polypropylene affects its mechanical properties and color performance in the composites formed.
U.S. Pat. No. 6,166,118 (and Great Britain Patent Application 232616A) describes an xe2x80x9cindirect pressurexe2x80x9d emulsification method, also called a xe2x80x9cdilution methodxe2x80x9d or a xe2x80x9cpressure dilution methodxe2x80x9d. In this method, the ingredients are heated in a pressure vessel with agitation to form a pre-emulsion concentrate. Hot water (or steam) is then slowly added with pressure to the pressure reactor to dilute the contents. However, the addition of water leads to cooling of the reactor and the pre-emulsion concentrate must, therefore, be reheated and kept at that elevated temperature for time sufficient to form an emulsion. The mixture is then cooled to form an aqueous emulsion. This method is disadvantageous in that it requires extra equipment or a facility for hot water (or steam) handling, is time consuming due to the necessity of having to re-heat the contents of the pressure reactor after the water has been added, and is potentially dangerous due the fact that hot water (or steam) must be added with pressure during the course of the emulsification.
Therefore, it is desirable to provide an aqueous emulsification of high molecular grafted polypropylene coupling agents in a one-step direct pressure process to overcome the disadvantages of the prior art. It is also desirable to provide an efficient way of applying high molecular weight grafted polypropylenes onto glass fiber surfaces during the glass fiber manufacturing process.
Accordingly, an important object of the present invention is to provide a method of emulsifying high molecular weight functionalized polyolefins that overcomes the disadvantages of the prior art.
It is another object of the present invention to provide an aqueous emulsion of functionalized polyolefins that have a molecular weight greater than 10,000.
It is yet another object of the present invention to provide a one-step direct pressure process for the aqueous emulsification of high molecular weight functionalized polyolefins.
It is a further object of the present invention to provide stable, low discoloring, high molecular weight functionalized polypropylene emulsion formulations.
It is another object of the present invention to provide aqueous emulsions of high molecular weight functionalized polyolefins suitable for direct application to a glass fiber surface during the glass fiber manufacturing process.
It is another object of the present invention to deposit high molecular weight grafted polyolefin directly on the glass fiber surface during the glass fiber manufacturing process.
It is a feature of the present invention that the high molecular weight functional polyolefin emulsion is free of alkyl phenol based surfactants.
It yet another feature of the present invention that the high molecular weight functionalized polyolefin emulsion is free of solvents and volatile organic compounds.
It is an advantage of the present invention that the high molecular weight functionalized polyolefin emulsion allows efficient direct interaction with the glass fiber surface.
It is a further advantage of the present invention that the high molecular weight emulsion is able to reduce oxidation and discoloration of the composite part.
It is an advantage of the present invention that the high molecular weight functional polyolefin emulsion is environmentally friendly.
It is yet another advantage of the present invention that little or no quantity of high molecular weight grafted polypropylene coupling agents in solid form is required during the subsequent compounding stage to obtain a high composite mechanical performance.
It is a further advantage of the present invention that less glass fibers are needed to achieve high performance in the composite product.
These and other objects, features, and advantages are accomplished according to the present invention by providing a one-step direct method of making a high molecular weight functionalized polyolefin aqueous emulsion. In this one-step direct method, a functionalized polyolefin having a molecular weight of at least 10,000, a fatty acid, a base, a surfactant, and water are heated in a pressure reaction vessel to a temperature above the emulsification temperature of the polyolefin with agitation for a period of time sufficient to form an aqueous emulsion. This high molecular weight polyolefin aqueous emulsion can be added to a sizing composition and added directly to glass fibers in the glass fiber manufacturing process.
The foregoing and other objects, features, and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description that follows.